In a wireless communication system, a transmitting terminal and a receiving terminal acquire a higher rate by means of spatial multiplexing and using multiple antennas. Compared with a common spatial multiplexing method, an enhanced technology is that the receiving terminal feeds back channel information to the transmitting terminal, and the transmitting terminal uses some transmission precoding technologies according to the acquired channel information, which largely enhances the transmission performance. In single-user Multi-input Multi-output (MIMO), the channel characteristic vector information is directly used for precoding; and in multi-user MIMO, relatively accurate channel information is needed.
In a Long Term Evolution (LTE) plan, the channel information is primarily fed back using a relatively simple single-codebook feedback method, while the performance of the transmission precoding technology for MIMO is more dependent on the accuracy of the codebook feedback therein.
The basic principle of the channel information quantization feedback based on a codebook will be set forth briefly as follows.
It is assumed that the limited feedback channel capacity is B bps/Hz, and then the number of available codewords is N=2B. After quantization, the characteristic vector space of the channel matrix forms a codebook space ={F1, F2 . . . FN}. The transmitting terminal and the receiving terminal jointly store or generate the codebook in real time (the same for the receiving terminal and the transmitting terminal). According to the channel matrix H obtained by the receiving terminal, the receiving terminal selects a codeword {circumflex over (F)} which best matches the channel from  according to a certain criterion, and feeds back a serial number i of the codeword to the transmitting terminal. The serial number of the codeword is referred to as a Precoding Matrix Indicator (PMI) here. The transmitting terminal finds a corresponding precoding codeword {circumflex over (F)} according to the serial number i, so as to obtain the channel information, wherein {circumflex over (F)} represents characteristic vector information of the channel.
Generally,  may be divided into multiple codebooks corresponding to Ranks, and each Rank correspond to multiple codewords, to quantize the precoding matrix formed by the channel characteristic vectors under the Rank. In general, there will be N columns in a codeword when Rank is N. Therefore, the codebook  may be divided into multiple sub-codebooks according to different Ranks, as shown in table 1.
TABLE 1 Number of layers υ (Rank)12. . . N  . . .  the set of codewordthe set of codewordthe set of codewordvectors when thematrixes when the matrixes when the number of columns number of columns number of columns is 1is 2is N
Wherein, when Rank>1, the codewords required to be stored are in a form of matrix, wherein such codebook quantization feedback method is used for the codebook in the LTE protocol. The codebook for 4 transmission antennas in the downlink for the LTE is shown in table 2. In practice, in the LTE, the precoding codebook has the same meaning as the channel information quantization codebook. In the following, for the purpose of uniformity, the vector may also be considered as a matrix with a dimension of 1.
TABLE 2CodewordTotal number of layers υ (RI)indexun12340u0 = [1 −1 −1 −1]TW0{1}W0{14}/ {square root over (2)}W0{124}/{square root over (3)}W0{1234}/21u1 = [1 −j 1 j]TW1{1}W1{12}/{square root over (2)}W1{123}/{square root over (3)}W1{1234}/22u2 = [1 1 −1 1]TW2{1}W2{12}/{square root over (2)}W2{123}/{square root over (3)}W2{3214}/23u3 = [1 j 1 −j ]TW3{1}W3{12}/{square root over (2)}W3{123}/{square root over (3)}W3{3214}/24u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1}W4{14}/{square root over (2)}W4{124}/{square root over (3)}W4{1234}/25u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1}W5{14}/{square root over (2)}W5{124}/{square root over (3)}W5{1234}/26u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1}W6{13}/{square root over (2)}W6{134}/{square root over (3)}W6{1324}/27u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1}W7{13}/{square root over (2)}W7{134}/{square root over (3)}W7{1324}/28u8 = [1 −1 1 1]TW8{1}W8{12}/{square root over (2)}W8{124}/{square root over (3)}W8{1234}/29u9 = [1 −j −1 −j]TW9{1}W9{14}/{square root over (2)}W9{134}/{square root over (3)}W9{1234}/210u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)}W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)}W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/ {square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
Wherein, Wn=I−2ununH/unHun in which I is a unit matrix, and Wk(j) represents a vector in the jth column of the matrix Wk. Wk. Wk(j1, j2, . . . jn) represents a matrix formed by j1, j2, . . . , jn columns of the matrix Wk.
The principle of the codebook feedback technology in the LTE is introduced above. In applications, some more specific feedback methods will be further referred. Firstly, a feedback granularity of the channel information will be introduced. In the LTE standard, the minimum feedback unit of channel information is subband channel information, and one subband is composed of a number of Resource Blocks (RBs). Each RB is composed of a number of Resource Elements (REs), and RE is a minimum unit of time frequency resources in the LTE. In the LTE-A, the resource representation method of the LTE continues to be used. A few Subbands may be referred to as Multi-Subband, and many Subbands may be referred to as Wideband. The feedback contents related to the channel information in the LTE will be described below. The feedback of the channel state information includes Channel Quality Indication (CQI for short) information, PMI and a Rank Indicator (RI for short). The CSI contents which are mostly concerned here are PMI information; however, both the RI and the CQI belong to the contents of the feedback of the channel state information. The CQI is an index for measuring whether the quality of the downlink channel is good. In the 36-213 protocol, the CQI is represented by integer values of 0-15, which represent different CQI levels respectively, and different CQIs correspond to respective Modulation and Coding Schemes (MCSs). The RI is used to describe the number of available spatial independent channels, which corresponds to the Rank of the channel response matrix. In the Open-loop spatial multiplexing and closed-loop spatial multiplexing modes, the UE needs to feed back the RI information, and in other modes, there is no need to feed back the RI information. The rank of the channel matrix corresponds to the number of layers.
Some mechanisms related to the channel information feedback in the LTE will be further described. There are two types of feedback modes for uplink channel information in the LTE, i.e., periodic channel information feedback in the Physical Uplink Control Channel (PUCCH) and periodic/aperiodic channel information feedback in the Physical Uplink Shared Channel (PUSCH). The PUCCH is a control channel, which has relatively high feedback reliability. However, the feedback resources thereof are relatively valuable, and the feedback overhead thereof is strictly limited. In general, the amount of CSI feedback in one feedback report (including one or more of PMI, CQI, and RI) can not exceed 11 bits; and if the amount of CSI feedback exceeds 11 bits, it will cause a sudden reduction in the performance. The PUSCH may provide relatively more CSI feedback resources; however, the reliability cannot be ensured, and it will influence the transmission of the data services since the resources for data transmission are to be occupied.
The Long Term Evolution Advanced (LTE-A) system, as an evolved standard of the LTE, supports a larger system bandwidth (up to 100 MHz), and is backward compatible with LTE related standards. In order to obtain higher average spectrum efficiency of a cell and improve the coverage and throughput of a cell edge, on the basis of the related LTE system, the LTE-A proposes some feedback enhancement technologies in terms of codebook feedback, which primarily enhance the feedback accuracy of the codebook and compress the overhead using the time correlation and/or frequency domain correlation of the channel information. The technology can enhance the spectrum utilization of the International Mobile Telecommunications-Advance (IMT-Advance) and relieve the shortage of spectrum resources. At the same time, in consideration that a more typical application is dual polarization, the enhancement of the codebook also takes a full consideration of the features of the dual polarization channel. The principle idea of the feedback technology of enhanced codebook increases the overhead of the PMI feedback compared to the feedback in the LTE, and the channel state information is represented jointly by feedback of two PMIs in two implementation ways primarily.
The dual-PMI feedback is defined, and the idea of the feedback way may be described as follows.
1) The precoding/feedback structure of one subband is jointly determined by feedback information of two PMI and corresponding codebooks. 2) The codebook is known previously by both the base station and the UE at the same time. The codewords corresponding to the feedback of the PMI may change at different times and in different subbands. 3) One PMI indicates the attributes of a wideband or a long-term channel. The other PMI indicates attributes of a subband or a short-term channel. 4) One matrix may be a fixed matrix, without feedback. At this time, it is equivalent to degenerate to single-PMI feedback (which may be used in cases of high-rank and low-rank uncorrelated channels).
It can be seen that a new structure based on dual-PMI feedback is proposed in terms of channel information feedback. For Rank=r, wherein r is an integer, the difference from the previous 4Tx codebook is that when single-codebook feedback equivalent to the dual-codebook is used, the feedback of the codewords in the corresponding codebook needs the feedback of two PMIs to represent the information thereof. The single codebook equivalent to the dual codebooks may generally be represented as the following table 3.
TABLE 3i2i101. . .N20Wi1,i2Wi1,i2Wi1,i2Wi1,i21Wi1,i2Wi1,i2Wi1,i2Wi1,i22Wi1,i2Wi1,i2Wi1,i2Wi1,i23Wi1,i2Wi1,i2Wi1,i2Wi1,i2. . .Wi1,i2Wi1,i2Wi1,i2Wi1,i2N-1Wi1,i2Wi1,i2Wi1,i2Wi1,i2N1Wi1,i2Wi1,i2Wi1,i2Wi1,i2
Here, Wi1,i2 is a codeword jointly indicated by i1 and i2, and may generally be written into a function form W(i1,i2), and there is only a need to determine i1 and i2. For example, when r=1, as shown in Table 4, illustrated is a codebook corresponding to LTE-A Rel12 rank1.
TABLE 4i2i1012345670-15Wi1,0(1)Wi1,1(1)Wi1,2(1)Wi1,3(1)Wi1+8,0(1)Wi1+8,1(1)Wi1+8,2(1)Wi1+8,3(1)i2i1891011121314150-15Wi1+16,0(1)Wi1+16,1(1)Wi1+16,2(1)Wi1+16,3(1)Wi1+24,0(1)Wi1+24,1(1)Wi1+24,2(1)Wi1+24,3(1)  Wherein  ,            W              m        ,        n                    (        1        )              =                                        1                          2                                ⁡                      [                                                                                v                    m                                                                                                                                                                  e                                                  j                          ⁢                                                      nπ                            2                                                                                              ·                                              e                                                  j                          ⁢                                                      mπ                            8                                                                                                                ⁢                                          v                      m                                                                                            ]                          ⁢                                  ⁢                  v          m                    =                        [                                                    1                                                              e                                      j                    ⁢                                                                                  ⁢                    2                    ⁢                    π                    ⁢                                                                                  ⁢                                          m                      /                      32                                                                                                    ]                T            
When r=2, a candidate codebook Option a, as shown in Table 5-a, is a first codebook corresponding to LTE-A Rel12 rank2.
TABLE 5-ai2i101230-15Wi1,i1,0(2)Wi1,i1,1(2)Wi1+8,i1+8,0(2)Wi1+8,i1+8,1(2)i2i145670-15Wi1+16,i1+16,0(2)Wi1+16,i1+16,1(2)Wi1+24,i1+24,0(2)Wi1+24,i1+24,1(2)i2i18910110-15Wi1,i1+8,0(2)Wi1,i1+8,1(2)Wi1+8,i1+16,0(2)Wi1+8,i1+16,1(2)i2i1121314150-15Wi1,i1+24,0(2)Wi1,i1+24,1(2)Wi1+8,i1+24,0(2)Wi1+8,i1+24,1(2)  Wherein  ,            W              m        ,                  m          ′                ,        n                    (        2        )              =                  1        2            ⁡              [                                                            v                m                                                                    v                                  m                  ′                                                                                                                          φ                  n                                ⁢                                  v                  m                                                                                                      -                                      φ                    n                                                  ⁢                                  v                                      m                    ′                                                                                      ]              ,            ϕ      n        =          e              j        ⁢                                  ⁢        π        ⁢                                  ⁢                  n          /          2                      ,            v      m        =                  [                                            1                                                      e                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      m                    /                    32                                                                                      ]            T      
When r=2, a candidate codebook Option b, as shown in Table 5-b, is a second codebook corresponding to LTE-A Rel12 rank2.
TABLE 5-bi2i101230-15Wi1+8,i1+8,i+24,i+24,0,0(2)Wi1+8,i1+8,i+24,i+24,0,2(2)Wi1+8,i1+8,i+24,i+24,2,0(2)Wi1+8,i1+8,i+24,i+24,2,2(2)i2i145670-15Wi1,i1,i1,i1,0,2(2)Wi1,i1,i1,i1,1,3(2)Wi1+8,i1+8,i1+8,i1+8,0,2(2)Wi1+8,i1+8,i1+8,i1+8,1,3(2)i2i18910110-15Wi1+16,i1+16,i1+16,i1+16,0,2(2)Wi1+16,i1+16,i1+16,i1+16,1,3(2)Wi1+24,i1+24,i1+24,i1+24,0,2(2)Wi1+24,i1+24,i1+24,i1+24,1,3(2)i2i1121314150-15Wi1,i1+16,i1+16,i1,0,2(2)Wi1,i1+8,i1+24,i1+24,i1+8,0,2(2)Wi1+16,i1,i1,i1+16,0,2(2)Wi1+24,i1+8,i1+8,i1+24,0,2(2)  Wherein  ,            W              m        ,                  m          ′                ,                  m          ″                ,                  m          ″′                ,        n        ,                  n          ′                            (        2        )              =                  1        2            ⁡              [                                                            v                m                                                                    v                                  m                  ″                                                                                                                          φ                  n                                ⁢                                  v                                      m                    ′                                                                                                                        φ                                      n                    ′                                                  ⁢                                  v                                      m                    ″′                                                                                      ]              ,            ϕ      n        =          e              j        ⁢                                  ⁢        π        ⁢                                  ⁢                  n          /          2                      ,            ϕ              n        ′              =          e              j        ⁢                                  ⁢        π        ⁢                                  ⁢                              n            ′                    /          2                      ,            v      m        =                  [                                            1                                                      e                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      m                    /                    32                                                                                      ]            T      
When r=3,4, a single codebook feedback technology is used, which uses codebooks of Ranks 3 and 4 in the Rel-8 4Tx. The use of the single codebook of Ranks 3 and 4 may be equivalent to considering that i1=0-15 and i2=0 or i2=0-15 and i1=0.
As in the LTE-A, the enhancement of the feedback accuracy makes the sum of overheads required for the feedback of the PMI1 and PMI2 information (which may also be construed as a total feedback overhead corresponding to W) increase relative to the overhead of the single codebook in the LTE, and when the codebook is applied to the feedback of channel information in the PUCCH and the PMI1 and PMI2 are transmitted in one subframe at the same time, the overhead of the CSI feedback of the PUCCH will exceed the overhead limit of 11 bits, which makes the transmission performance of the PUCCH degrade seriously, and seriously influences the system. In addition, even if the PMI1 and the PMI2 are transmitted separately, for example, the PMI1 and the RI are transmitted at the same time, although the feedback of the PMI+RI does not exceed the overhead limit of 11 bits, as the increase in overhead will cause an increase in the bit error rate in a case that the transmission resources are fixed, the bit error rate of the RI cannot be well ensured, and there is thus a problem that the bit error rate of the RI cannot meet requirements. When the wideband PMI2 and the wideband CQI are transmitted together, an excessive overhead will results in an increase in the bit error rate, which influences the performance of the system. Finally, when the subband PMI and the subband index as well as the subband CQI are transmitted together, the overhead limit of 11 bits will also be exceeded, which seriously reduces the performance. The above problem is an important technical problem to be solved in the related art.